There are many types of electric machines (motors and/or alternators) in the prior art. Each type of electric machine has various advantages and disadvantages for any particular application. Therefore, the type of machine used typically depends on the application. One type of electric machine is a brushless permanent magnet DC machine. A typical brushless permanent magnet machine will have a relatively high power density because of its high-efficiency and because most losses occur in the stator, which is much easier to cool then a rotor. Expensive, rare earth magnets can provide a stronger magnetic field that can significantly increase the power density of such motors, but also result in a higher manufacturing cost.
In some applications of DC motors, a high torque, low speed motor is desired. For example, motors for a traction drive system require high torque, low speed motors. Each of these attributes can be enhanced by increasing the pole count of a motor. However, as mentioned above, rare earth magnets, as well as other components of the motor, make increasing the pole count expensive.
Typical brushless motors require a control system providing a sophisticated switched mode power amplifier or a three phase sinusoidal inverter (when driven by sinusoidal excitation, brushless permanent magnet machines are often called permanent magnet synchronous machines). The power electronics required to power and control these types of machines are typically the most costly component of a resulting system. Therefore, despite the machines desirable performance, such machines have not been widely applied in cost sensitive applications.
There is therefore a need for low cost brushless DC machines capable of providing high torque and low speeds.
It is therefore an object of this invention to provide a dual stator axial gap DC brushless machine where the stators are angularly offset with respect to one another and the rotor disposed between stators has a pole count greater than each stator.